slight discrepancy between implementation and bip

Comments:BIP-xyz.md

@@ -0,0 +1,20 @@
+I already reported on #lightning-dev but worth documenting here in case anybody else is playing around with it, this blank `prevout` at 
+https://github.com/cdecker/bitcoin/blob/noinput/src/script/interpreter.cpp#L1217  
+is used when `SIGHASH_NOINPUT` is the sighash type, but the input index that's being set is actually `0xFFFFFFFF` instead of `0x00000000` as the bip suggests, can be seen here :  
+https://github.com/cdecker/bitcoin/blob/noinput/src/primitives/transaction.h#L24  
+Something like the following will have to be added for the implementation to be compatible with the bip
+```diff
+diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp
+index 3df1fc0c9..43e398581 100644
+--- a/src/script/interpreter.cpp
++++ b/src/script/interpreter.cpp
+@@ -1230,6 +1230,8 @@ uint256 SignatureHash(const CScript& scriptCode, const CTransaction& txTo, unsig
+         if (!noinput) {
+             prevout = txTo.vin[nIn].prevout;
+             script = scriptCode;
++        } else {
++            prevout.n = (uint32_t)0;
+         }
+
+         if ((nHashType & 0x1f) != SIGHASH_SINGLE && (nHashType & 0x1f) != SIGHASH_NONE) {
+```